Synchronizing system



April 1937- J. w. HORTON ET AL 2,075,898

SYNGHRONIZING SYYSTEM Original Filed Junel1 1925 3 Sheets-Sheet 1 April 6, 1937- J. w. HORTON ET AL 2,075,898

SYNCHRONI Z ING SYSTEM Original Filed June 11, 1925 5 Sheets-Sheet 2 mum/0m.- Jase 0 W170i)? 0004/4/51 Terry April 3 J. W. HORTON ET ,AL. 2,075,898

SYNCHRONI Z ING SYSTEM Original Filed June 11, 1925 3 Sheets-Sheet 3 LIGHT VALVE hue/vars:

Jasepfi 14/ 174/700 0004/0 M Terry by My in the transmission Patented Apr. 6, 1937 UNITED STATES PATENT OFFICE M. Terry, New York, N.

Electric Company,

Y., assignors to Western Incorporated, New York,

N. Y., a corporation of New York Application June 11, 1925, Serial No. 36,357 Renewed September 5, 1930 19 Claims.

This invention relates to control devices and more especially to the means for controlling the movement of synchronously operating members, such as employed in picture transmission systems and telegraph systems.

In order that the proper sequence and arrangement of signals sent from a transmitting station at a receiving station, it has purpose of scanning -of the entire The transparent picture to allowing a progressive area of the picture. be transmitted and positioned upon their respective drums'that there is a portion of each drum which is not effective of impulses corresponding to the picture characteristics. As the sending and receiving drums are rotated, these blank portions are presented to the light source once during each revolution of the respective drum. The portion of a revolution during which the blank portions of the drum are thus presented, is termed the underlap period".

In the transmission of pictures it is essential, therefore, that the sending and receiving drums be continuously in phase and in synchronism, in order to avoid distortion in the reproduced pic- For this purpose a driving motor at the station. In a preferred embodiment of the invention there is employed a phonic wheel or a tuning fork controlled motor for driving both the sending and receiving drums, eachdriving motor having associated therewith another motor for synchronizing purposes. The tuning fork associated with the driving and synchronizing motors has associated therewhich is adapted to be energized during the underlap period by means of correcting impulses. The energization of this damping magnet serves to control the synchronizing speed of the associated motors.

It is an object of this invention, therefore, to control the speed of driving members employed in signaling systems.

A feature of the invention relates to a signaling system wherein continuously operable driving elements at sending and receiving stations are the recording film are so controlled by means of carrier current of characteristic amplitude.

Another feature relates to a picture transmission system wherein special motors at the sending and receiving stations cooperate to maintain 5 synchronous speed of the driving motors at each i of said stations.

A still further feature relates to a speed controlling mechanism which is controlled jointly by apparatus at both ends of a transmission line to maintain synchronous speed of driving elements at each end thereof.

Other features and advantages not specifically enumerated will become apparent aftera consideration of the following description and the appended claims.

Referring to the drawings, Fig. 1 shows circuits. and apparatus at a sending station comprising driving and synchronizing motorscontrolledby a tuning fork; a source of light and a light sensitive cell; together with means for allowing the picture to be scanned. This figure also represents schematically by means of rectangles variousmodulating and amplifying equipment, as well as an outgoing transmission line. 25

Fig. 2 discloses substantially similar apparatus to that shown in Fig. 1 ,and is located at the incoming end of the transmission line.

Fig. 3 illustrates a modification of the invention as applied to a picture transmission system. 30

Fig. 4 is a curve showing the variations of the transmission line current at different periods of time.

While it has been chosen to illustrate the invention as applied to a system for transmitting 35 pictures, it is understood that in its broad aspects the invention is not to be so limited. It will be apparent, that the invention may be applied to any signaling system wherein rotatable elements atreceiving and sending stations are required to operate in synchronism.

The invention as disclosed in the drawings is shown as applied to a system of picture transmission similar to that described in Patent No. 1,606,227, granted Nov. 9, 1926 to J. W. Horton, H. E. Ives and M. B. Long.

Referring to Fig. 1, there is shown a driving motor I, which may be of any type adapted tooperate at a relatively constant speed. This motor is preferably a phonic wheel or tuning fork controlled motor, and may be started in operation manually. A similarmotor 35 is also provided for synchronizing purposes, as will appear hereinafter. When magnet 20 receives a starting impulse, the springactuated clutch member I8 is released. The clutch member thereupon cooperates with gears l0 and 24, whereby gears l6 and 25 are rotated. Mounted on the shaft 26, rotated by gear |6, is a drum I5, adapted to rotate with said shaft. The frame 84 in which the shaft I2 is rotatably mounted is adapted to move longitudinally along the base plate 93, when thread |I rotates in threaded bearing 94. Gear 25 by means of the sleeve 92 is so mounted on shaft as to move longitudinally and integrally with the frame 84. Thus when shaft 95 rotates,

rotatory and longitudinal movements. are imparted to gears Hi and 25.- The gears l6 and 25 are so designed as to allow for the proper ratio between the rotational and longitudinal movements of drum l5.

The picture to be sent is mounted on drum l5, and as the drum rotates, light from the source 2| is projected by means of lens 21 on the drum and passing through the transparent portions of the picture affects the photoelectric, or other light sensitive, cell 22, which has its resistance varied in accordance with the characteristics of the picture. connected to the-input circuit of amplifier A so that the variations in the resistance of the cell cause corresponding variations of current flow in the input and output circuits of the amplifier. The output circuit of amplifier A is connected to the modulator M, whereby the amplitude of the carrier waves generated by oscillator O are modulated, and are then impressed on the line L after passing through the transmitting terminal amplifier TTA. For a detailed description of the circuits and the method of operation of modulator M, reference may be had to the patent to Messrs. Horton, Ives, and Long, referred to hereinabove.

The picture is so mounted on drum |5 that between the ends of said picture there is an opaque portion 23 which intercepts the rays of light from the source 2| for a comparatively short period, called the underlap period, during each revolution of drum |5. During this underlap period, therefore, the carrier waves are not modulated in accordance with the picture characteristics, but are changed in amplitude for synchronizing purposes, as will be described hereinafter.

From an inspection of Fig. 1, it will be noted that during the period of transmission of the picture characteristics that the lower right winding 5| of transformer 52 alone is effective to cause the carrier waves to be modulated, thus allowing a maximum amplitude to the modulated waves, represented by amplitude of portion A of the curve of Fig. 4. However, during the underlap period, both right windings 5| and 53 are effective, thereby increasing the amplitude of the carrier waves beyond their amplitude during the picture transmission period.

At the receiving station shown in Fig. 2, the modulated carrier waves are received over line L and are amplified by receiving terminal amplifler ETA and picture amplifier PA, after which the impulses affect the light valve. II, or they may be employed in any other manner for reproducing the picture. motor 96, which is preferably similar to motor I4 is employed for rotating shaft 81, which is suitably geared to the crown wheel 28. A drum 29, similar to drum I5, is rotated by gear 30 when the spring actuated clutch member 3| is released byactuation of the start magnet 99. The portion 33 of the drum 29 included between lib? nds of The cathode and anode of this cell are As shown, the drivingthe picture corresponds to the portion 23 of drum 5.

, In order to avoid distortion in the received picture, it is necessary that drums |5 and 29 rotate in synchronism. For the purpose of maintaining accurate synchrcnism of the sending and receiving drums, there is provided at the sending station a motor 35 controlled by thefork 86, which also controls motor I4. By means of this arrangement motors 35 and M will operate in synchronism. A similar arrangement exists at the receiving station, fork 68 controlling the synchronous operation of motors 54 and 96. Each of the controlling tuning forks 86 and 68, once motion is imparted thereto, continues to vibrate at its natural period under control of a self-interrupting circuit. For example, contact 88 provides a self-interrupting circuit for the fork 86. This contact when closed completes a circuit from battery through the winding of the driving magnet 89. Magnet 89 is, therefore, energized and deenergized in correspondence with the natural frequency of the tines of fork 86. Each of the forks is also provided with sets of contacts for controlling the operation of the associated motors I4, 35 and 96, 54, in the well known manner. The tuning fork 68 located at the receiving station isalso provided with a damping magnet in a manner to be described hereinafter.

Motor 35 at the sending station controls a shaft comprising sections 36, 31, etc., which are insulated from each other by means of the insulating segments 38 and 58. Rigidly secured to this shaft are a number of disc-like commutators 39, 40, 4|, 42 and 43. The shaded portion of each disc represents insulation and the unshaded portion represents conducting material. Arranged for cooperation with the commutators are brushes 45, 46, 41, 48 and 49. 'It will be noted, therefore, that a circuit is prepared by means of brush 46 and commutator 40 during the entire revolution of the shaft. Brushes 45, 48 and 49 are so positioned that they make contact with the conducting portions of their respective-commutators only during that portion of a revolution which corresponds to the underlap period of drum l5. Similarly brush 41 is positioned so as to be insulated.

from the shaft section 31, only during the underlap period. The angular width of the conducting segments on commutators.39, 42 and 43, and of the insulating segment of commutator 4| may be such as to provide commutation during a complete underlap period or during only a portion of this period as may be necessary for synchronizing purposes.

lap period brush 41 rests on an insulating segment and consequently the carrier waves are unmodulated. Since both windings of transformer 52 are thus effective during the underlap period, the carrier current impressed on line L will have an amplitude represented by the portions B of the curve of Fig. 4.

, During the major portion of the revolution of drum l5, and assumingthat no picture signals 41 and commutator 4|, through the output winding of modulator M, thence to the lower conductor of the line. The carrier current impressed on the line L, by means of winding 5|, has a constant amplitude represented by the portions A of the line current curve. When pictures are being sent, however, the current A is modulated in accordance with'the picture characteristics, as represented by section C of the line current curve.

The receiving station is provided with a synchronizing motor 54, corresponding to motor at the sending station. Motor 54 controls a shaft having portions insulated from each other by means of the insulating segment 96. The portion 55 carries the commutator discs 56 and 51, having conducting segments 58 and 59, respectively, which are in electrical communication with the shaft section 55. Arranged for cooperation with the gimmutators 56 and 51 are brushes 9'! and 52 passes through the rectifier R, and the upper windings of relay 6|, causing said relay to attract its armature 66 to close its contacts. Shortly thereafter a circuit is closed from the negative D016 of battery 66, through the lower windings of relay 6|, brush 96 66. The energization of the lower windings of relay 6| is suflicient to overcome the attraction due to the energization of the upper winding and relay 6| releases its armatures to open the confor any reason decrease in speed, due to a decrease in the rate of vibration of fork 68, the period of closure of the contacts of relay 6| increases. Similarly, should motor 54 increase in speed, because of an 68, this period is correspondingly shortened.

During the time that relay 6| is operated, con

denser 12 is charged by means of a circuit including battery 13, closed contacts of relay 6| and resistance 14. The condenser 12 thereupon con tinuously discharges through resistance 15, which values of condenser 12 and resistances 14 and I5,-

and an average current flows through the windincrea'se of vibration of fork ing of magnet 69. It will be noted that increased energization of magnet 69 has a similar effect to a mechanical stiffening of the tines of the fork which thereupon vibrate at a faster rate, the converse being true when magnet 69 has a decreased current flowing therethrough.

Should fork 68 tend to vibrate at a slower rate than fork 86, motor 54 lags and the period of closure of the relay contacts increases as described above, and the mean potential on the grid of tube ll increases, thus increasing the space current of said tube and the current through the winding of magnet 69, allowing the tines of fork 68 to vibrate faster. On the other hand, should motor 54 increase in speed, due to an increase in the vibration rate of fork 68, the mean potential on the grid decreases and a decreased current flows through the-tube II and the winding of magnet 69 to-correspondingly dampen the vibrations'of fork 68.

When it is desired to transmit pictures, tuning forks 66 and 68 are initially set in motion, motor 35 at the sending station and motor 54 at the receiving station may be started in operation.

Shafts 36 and 55 are thus rotated, and unmodulated carrier current generated by oscillator O, is impressed on the line by means of the winding 5| of transformer 52. After an interval due to the action of relay 6| and tube H as above explained motors 35 and 54 are synchronized,

whereupon motors [4 operation.

Key 16 is next 18. If this key and 96 may be started in operated to close contacts 11 and is thrown during the occurrence of the underlap period of drum l5, a circuit is completed from positive pole of battery 8|, through thecontacts of key 85, winding of clutch magnet 26, conductor 86, contact 18 of key 16, brush 48 and conducting segment of commutator 42, conducting segment of commutator 43 and brush 49, to the negative pole of battery 8|. Magnet 26 immediately locks under control of key 85. .As soon as key 16 is operated during the underlap period, the output circuit of the modulator is shunted by means of a path traceable from the upper terminal of the modulator output coil, condenser 19, contacts 11 and 18 of key 16, brush 48 and conducting segment of commutator 42, conducting segment of commutator 43 and brush 49, to the lower terminal of the modulator outputcoil. Condenser 19 is of such capacity as to reduce the line current to substantially zero value, as indicated by the portions-D of the line current curve.- With magnet 26 locked should key 16 be held operated after the occurrence of the underlap period, the modulator output coil remains shunted over a path traceable from the upper terminal'of said output coil, condenser 19, contacts 11 of key 16,'conductor 86, looking contacts of magnet 26, to the lower terminals of the modulator'output coil. The reduction of current in' the line as above described serves to set up the receiving apparatus in readiness for the modulated carrier by operating magnet 99, as is fully described in the patent to Messrs. Horton, Ives and Long, referred to hereinbefore. When magnet 26 energizes, it immediately locks under control of the key 85. The pivoted arm I 9 is attracted, releasing the clutch member l8, whereby the drum I5 is rotated. The above mentioned lockingcircuit-for magnet 26 is maintained until the drum l5 has reached the limit of its longitudinal movement, whereupon a projection on'the drum carriage engages a trip finger on key 85 to open the contacts thereof and release the clutch magnet 26; Should key 16 be'maintained operated for more than a single revolution of the drum I then when brushes 49 and 49 next make contact with their cooperating conducting segments the condenser 19 short-circuits the output 5 terminals of the modulator M. The synchronizing current is however applied to the line L by means of both windings oi transformer 52 and brushes 45 and 46 as hereinbefore described.

Should the brushes 8 and 49 be in contact with 10 their conducting segments that is, during the underlap position of the drum, when'key 16 is first operated, then the modulator is short-circuited as above described and no modulated current fiows in the line L until the underlap position is passed when brushes 46 and 41 complete a' circuit through their respective commutators. At the receiving station the interruption of the carrier current caused by the operation of the key I6 brings about the energizatlon of magnet 99 as 20 described in the patent to Messrs. Horton, Ives and Long referred to hereinbefore. Magnet 99 attracts the pivoted arm 92 whereby the clutch member 3| is released to cause the rotation of gear 30 and drum 29. When key 16 at the sending station is restored to normal the shunt circuit is removed from the output terminals of the modulator and carrier waves modulated inaccordance with the picture characteristics are impressed on line L producing a line current of the general character represented by portion 0 of the curve of Fig. 4.

In the form of the invention illustrated in Fig.

3, a separate carrier wave of characteristic frequency is adapted to be impressed on the line, during the underlap period, for synchronizing purposes. In this case motors A and B for driving the picture transmitting drum and synchronizing commutators I91 and I98 respectively, at the sending station, arecontrolled by the tuning fork I90. Carrier current generated by the oscillator I9I is modulated and amplified by means of the modulator-amplifier I93 in accordance with the picture characteristics. After passing through the filter I94 the modulated current is impressed on line L. In a similar manner to that already described in connection with Fig. 1, at a predetermined interval during the revolution of the sending drum, preferably during the underlap period, as determined by the relative positions of brushes cooperating with commutators I91 and I98, carrier current of a special frequency generated by oscillator I95 is applied to line L.

At the receiving station represented in the right-hand portion of Fig. 3 the picture current and the synchronizing cugent are separated by filters I99 and I00, respectively. Thecarrier current generated by oscillator I95 will be passed by filter I00 and applied to the upper windings of relay IOI after passing through the rectifier I02. As a result of this arrangement since the special synchronizing current exists on the line only during the underlap period, relay IOI neednot be a marginal relay. Relay IOI corresponds to relay SI of Fig. 2. The action of relay IM and the operation of the discharge device I0! is similar to that already described in connection with corresponding apparatus of Fig. 2. The synchronizing operation of tuning fork I 05 is in this case controlled by magnet I09. The modulated carrier current generated by oscillator I9I is passed by filter I99 and functions in the reproduction of the picture in any desired manner. The forms of the invention which have been described hereinbefore represent the tuning fork as a source of 'constant impulses for controlling the driving means but it is evident that any similar source providing for definitely recurring impulses could be employed. A source of alternating current can thus be substituted for the tuning forks as it is evident that the main requirement of the system is synchronization of the sending and receiving drums, since the speed of the system may vary within wide limits without affecting the picture reproduced.

What is claimed is:

l. A signaling system comprising a sending station, a receiving station, continuously movable means at said sending station adapted to operate in synchronism with corresponding continuously movable means at said receiving station, means controlled jointly by said sending and receiving stations for determining the synchronous operation of the continuously movable means, said last-mentioned means being responsive only to signaling current of characteristic amplitude.

2. In a carrier current transmission system, a sending station, a receiving station, means for generating a sustained alternating carrier current, means for modulating said carrier current by signalling currents, continuously rotatable elements at both of said stations, continuously vibrating means at said receiving station for controlling the rotatable elements thereat, a magnet for controlling the operation of said vibrating means, means eiiective at regularly recurrent intervals to increase the amplitude of said carrier current above the maximum signaling amplitude, and means responsive to said increased carrier current for varying the control of saidmagnet over said vibrating means to correspondingly vary the speed of the rotatable elements at the receiving station.

3. In a carrier current transmission system, a sending station, a receiving station, continuously operable driving motors and speed control means therefor at each of said stations, said speed control means being normally efiective to maintain synchronous operation of said motors, means for modulating a carrier current in accordance with signals to be transmitted, means effective at regular intervals for increasing the amplitude of said carrier current and means at said receiving station responsive to said increased current and effective when said motors are running asynchronously to restore the synchronous operation of said motor adapted to be closed for a certain period I during each revolution thereof, a relay for maintaining synchronous operation of said motors, a circuit through said relay comprising a switch associated with one of said motors, another circuit through said relay comprising a switch associated with the other of said motors, the period of joint closure of both of said circuits being constant when said motors are running synchronously, and means effective when said motors are running asynchronously for correspondingly varying the period of closure of said circuits to cause said motors to run synchronously.

5. In a carrier current transmission system a sending station, a receiving station, a driving motor 'at each of said stations, a synchronizing motor at each of said stations, switches controlled by each of said synchronizing motors, speed control means associated each of said mators, a magnet for the speed control means at the receiving station to maintain the motors thereat in synchro- 6. In a signaling system, a sending station, a

receiving station, a continuously operable motor at each of said stations, a continuously operable synchronizing motor at each of said stations, speed control means at each station common to said motors thereat, an operating circuit for said control means adapted to be closed by one of said synchronizing motors, a, releasing circuit for said control means closed by the other of said synchronizing motors, means effective when said motors are running synchronously for maintaining the efiect of said operating and releasing circuits constant, and means effective when said motors are running asynchronously for varying the relation between the effect of the said operating and releasing circuits to restore the synchronous operation of said motors.

7. In a carrier current signaling system, a sending station, a receiving station,'a continuously operable driving motor and a synchronizing motor at each of said stations, a relay common to said synchronizing motors adapted to be operated during a particular portion of each revolution of one of said synchronizing motors and adapted to be released for a particular por-' tion of each revolution of the other of said synchronizing motors, means efiective when said driving motors are running asynchronously for varying the periods of operation and release of said relay to restore the synchronous operation of said driving motors.

8. In a picture transmission system, a sending station, a receiving station, continuously operable driving means at each of said stations, means for sending carrier current modulated in accordance with the shade of successive points of successive linear elements of a picture, means efiective between the successive linear elements for increasing the amplitude of said carrier current to control the synchronous movements of said continuously operable driving means.

9. In a picture transmission system, a transmission line, a plurality of stations, continuously movable means at said stations adapted to be operated in synchronism, means for scanning a picture in successive linear elements, means for sending current corresponding to said linear elements, means efl'ective at the end of the scan ning of each linear element for increasing the amplitude of said current, a, marginal relay at one of said stations operated in response to said increased current for increasing or decreasing the speed of said movable means at said station.

10. A picture transmission system comprising a sending station, a receiving station, means-for sending to said receiving station current varying in amplitude according to the characteristics of the picture, light controlling means at said receiving station responsive to said current for controlling the tone values of the picture to be produced, means for maintaining synchronism between continuously operable apparatus at said stations comprising means at the sending station for intermittently transmitting current impulses of a fixed amplitude difl'erent from the istics, and means tics for maintaining amplitudes corresponding to the picture characteristics, and means independent of said light controlling means at said receiving station responsive to said impulses for controlling the synchronous operation of the continuously operable apparatus at the sending and receiving stations.

11. An image producing system comprising a transmitting station and a receiving station, a source of carrier waves, means comprising a movable element for modulating waves from said source for giving them certain characteristics corresponding to the tone values of the elemental areas of a field of view an-image of which is to be produced, switching means for periodically modulating the waves from said source to give them a characteristic different than said first mentioned characteristics, and means at said receiving station for producing an image of said field of view, said means comprising a continuously movable element, means for producing light having variations determined by the carrier 'waves having said first mentioned characterindependent of said light pro-- ducing means responsive to the carrier waves of said second mentioned different characterissaid movable elements in synchronism.

12. An electro-optical image producing system comprising means including a continuously movable element for scanning an object in a series of parallel lines to produce an image current having variations corresponding to the tone valthe object, means including a second continuously movable element I saidobject under con-5" trol of said image current, means for intermit ues of elemental areas of for producing an image of tently producing synchronizing current having .a characteristic different riod required for a single, complete scanningof from the .characteristics' of said image current at'intervals during the pethe object, and'means responsive only to said synchronizing current for maintaining said continuously movable elements in synchronism.

13, An electro-optical' image producing system parallel lines to produce an image current having variations corresponding to the tone values of elemental areas of an object, means including a second continuously movable element for producing an image of said object under control of said image current, means for intermittently producing synchronizing current at intervals separated by the line scanning period, and means responsive only to said synchronizing current for maintaining said continuously movable elements in synchronism.

14. An electro-optical image producing system comprising means for producing an alternating carrier current, meansincluding a continuously movable element for scanning an object in a series of parallel lines modulating current from said source of altemating carrier current, means including a second continuously movable element for producing an image of said object under control of said modulated carrier current, and means for intermittently producing alternating synchronizing current at intervals separated by the line scanning period for maintaining said in synchronism.

to produce an image current for continuously movable elements sponding to the tone values of elemental areas of an object and synchronizing portions occurring only between successive image portions, means for producing light the intensity of which is controlled by current in said path for controlling the tone values of an image of said object, and continuously movable image synthesizing means the speed of which is controlled only by the synchronizing portions of said current independently of said light producing means.

16. Electra-optical image producing apparatus comprising a current path carrying a single modulated carrier current having time separated image portions corresponding to the tone values of elemental areas of an object and synchronizing portions having an amplitude greater than the amplitude of said image portions, occurring only between successive image portions, two circuits energized under control of current in said path, light controlling means in one of said circuits for controlling the tone values of an image of said object in accordance with the variations of said image portions of said modulated carrier current, continuously movable image synthesizing means, and means independent of said light controlling means for controlling the speed of said continuously movable image synthesizing means under control of the synchronizing portions of said modulated carrier current.

17. Image producing apparatus comprising a current path carrying a single current having time separated image portions corresponding to the tone values of elemental areas of an object and synchronizing portions having a difierent characteristic occurring only between successive image portions at intervals less than a complete scanning period, and means for setting up an image element by element comprising scanning means which scans with a continuous movement successive strips of the image in periods between which there are intervals when said movement is ineffective in the setting up of the image, means for producing the light variations of the image under control of the image portions of said cur- 45 rent, and means under control of the synchronizing portions of said current for acting upon said scanning means to produce an acceleration or retardation of the scanning movement to 'correct for lack of synchronism with the transmitting apparatus.

18. Image producing apparatus comprising a current path carrying a single current comprising time separated image portions corresponding to the tone values of elemental areas of an object and synchronizing portions having a different characteristic occurring only between successive image portions at intervals less than a complete scanning period, and means for setting up an image element by element comprising scanning means which scans with a continuous movement repeatedly across the receiving field and beyond said field in an overlap region, means for producing the light variations of the image under control of the image portions of said current, and means under control of the synchronizing portions of said current operating while said overlapping region is being scanned for acting upon said scanning means to produce an acceleration or retardatlon of the scanning movement to correct for lack of synchronism with the transmitting apparatus.

19. Image producing apparatus comprising a current path carrying a single current comprising time separated image portions corresponding to the tone values of elemental areas of an object and synchronizing portions of greater amplitude occurring only between successive image portions at intervals less than a complete scanning period,

and means for setting up an image element by element comprising scanning means which scans with a continuous movement successive strips of the image in periods between which there are intervals when said movement is ineffective in the setting up of the image, means for producing the light variations of the image under control of the image portions of said current, and means under control of the synchronizing portions of said current for acting upon said scanning means to produce an acceleration or retardation of the scanning movement to correct for lack of synchronism with the transmitting apparatus.

JOSEPH W. HORTON. DONALD M. TERRY. 

